I have a Mist wallet and a contract (with its address) that i execute from mist GUI "manually".
I was wondering how i can execute it from command line (bash not geth) or call that execution from a python script.
If this is possible could you provide a toy example or a roadmap?
Thanks in advance!

Executing/calling from bash

From the very basics, Ethereum has an RPC (Remote procedure call) interface, as explained excellently by the Wikipedia RPC page. Ethereum's RPC interface uses a subset of the JSON-RPC 2.0 spec, with the following caveats:

Numbers are hex encoded.

If the RPC method takes a block number, you can submit either an actual number, or one of the following strings: "earliest", "latest", "pending".

Commands with bash

Using the RPC interface, we can execute commands such as eth_coinbase, which returns your ETH address:

Interacting with deployed contracts with bash

Functions from pre-deployed contracts are interacted with through their function signature, which is found by taking the first 4 bytes of the output of sha3('functionName(functionParameterTypes)'). This can be computed from inside geth with, for example,

> web3.sha3("multiply(uint256)").substring(0, 8)
"c6888fa1"

A helpful note here is that even if you've used uint or int within your contract, you'll have to use the real deal like eg uint256.

You pad your input (encoded in hex) with zeros, like the following, for an example input of 6:

0000000000000000000000000000000000000000000000000000000000000006

You do this so the length is a multiple of 32 bytes, but usually I just copy the example and change the end so as not to have to hit the 0 button 8 billion times.